Thermal separation processes like evaporation allow valuable products in a liquid mixture to be separated or enriched. Different fuel qualities, pharmaceutical grade glycerine, ethanol or solvents like acetone are typical examples of evaporation products.
However, some of these high-boiling products are very sensitive to heat and cannot be handled using conventional evaporators. In this case, they can be treated by means of short path evaporation.
How short path evaporators work
Pictures 1 and 2 illustrate the working principle of a short path evaporator, which belongs to the category of thin film evaporators. It operates at very low absolute pressure of between 1 and 0.001 mbar in order to keep the evaporation temperature as low as possible.
The liquid mixture is fed to the top of the rotor and distributed to the machine’s heated jacket surface. The rotor creates a thin, mechanically agitated product film on the heating surface inside the body. The product flows downwards in a spiral path due to gravity; the volatile portion of it evaporates in the process. The vapour takes the shortest route and reaches the internal condenser with practically no pressure loss. The non-volatile portion moves to the lower part of the evaporator and is discharged through the bottom outlet. The residual vapours and inert gases flow through the vacuum nozzle to the vacuum system. The distillate – the condensate from the condenser – is removed as a liquid, also at the bottom of the evaporator.
Useful for a variety of applications
Short path evaporators offer very good results with evaporation, concentration, distillation or degassing of high-boiling, temperature sensitive products. This type of evaporator is therefore very useful in applications like:
- stripping isocyanates from pre-polymers
- evaporating oil and wax from petroleum
- distilling monoglycerides from di- and triglycerides
- concentrating Omega-3 fatty acids
- fractionating waxes into hard and super hard waxes
- concentrating vitamin E (tocotrienol and tocopherol)
To match the requirements of all these applications, the rotor design has to be adjusted in order to obtain high-quality end products. SMS can supply short path evaporators with several different rotor types for this reason. They are manufactured in a hygienic design optionally using PTFE or graphite wiper elements, spring loaded PTFE wiper elements, metal wiper elements or polymer or graphite rollers. In addition, the rotors can be equipped with specially designed splash guards, protecting the internal condenser from droplets in the vapour stream and ensuring high distillate quality.
Focus on liquid distribution
One of the design challenges with short path evaporators is the distribution of the liquid feed on the internal heating surface. Owing to the working principle, the liquid has to be fed from the top of the evaporator, i.e. from the top of the rotor, and then distributed as uniformly as possible on the internal heating surface. Non-uniform distribution can result in the heated surface not being covered with feed material as well as in unintended splashing. Heated surfaces which are not used for the evaporation process increase the evaporator size and therefore the cost of the equipment. Severe splashing creates droplets which can get into the distillate and lead to a product that is not properly mixed. Both these effects should be avoided in the interests of optimum results.
Due to the importance of the liquid feed distribution on top of short path evaporators, Buss-SMS-Canzler (SMS) developed an optimised liquid distribution system. This system can be designed according to the process requirements, for example the material properties of the feed, the operating temperature and various other parameters. Picture 3 shows the liquid flow on the top cover of the rotor of a short path evaporator; it was calculated using CFD (computational fluid dynamics). The flow field on the left was calculated for the former, conventional SMS design. The liquid only leaves the distributor in a very limited section of the circumference, so that part of the internal heating surface is not used for evaporation.
The flow field on the right-hand side exhibits a very uniform liquid pre-distribution all around the circumference. After reaching the internal heating surface, the entire liquid can be distributed evenly by the wiper blades. This results in a very uniform, thin film and optimum usage of the internal heating surface. After experimental verification, the CFD computations were translated into design equations and the optimised liquid feed distributor incorporated in the standard SMS rotor design.
Online search: cpp0218Buss-SMS-Canzler
Hall 4.0, Booth B24
AutHor: Dr. Bernd Genenger
Head of sales & marketing, Buss-SMS-Canzler
